1. Field of the Invention
The present invention relates to an improved etching method, a process for producing a semiconductor element using said etching method, and an etching apparatus suitable for practicing said etching method. More particularly, the present invention relates to an improved etching method which is simple and excels in selective etching precision and which enables etching an object to be etched into a desirable state with no or very slight damage at the non-etched region or layer thereof. The present invention also relates to a process for producing a semiconductor element based on said etching method, which comprises a small number of steps and enables efficient production of semiconductor element by way of a fast etching treatment, at a reduced production cost, by this method, a transparent and electrically conductive film of a photovoltaic element as the semiconductor element can be patterned in a desirable state while repairing defects such as short circuit defects of the photovoltaic element. The present invention further relates to an etching apparatus suitable for practicing the above method and process.
2. Related Background Art
In recent years, etching techniques have been widely used during the production of various semiconductor elements used in photovoltaic elements including solar cell elements, photodiodes, and the like. For instance, in the production of photodiodes and ICs (integrated circuits), the etching technique has been used in patterning or removing an electrode comprising a metal electrically conductive film or transparent and electrically conductive film, a base member or a semiconductor layer. Further, in the production of semiconductor elements such as solar cell elements, the etching technique has been used in patterning or removing an electrode comprising a transparent and electrically conductive film or a semiconductor layer. Besides these, patterning by way of etching a transparent and electrically conductive film has been used in the production of displays such as liquid crystal panels and the like.
Particularly, in the production of an amorphous silicon solar cell, there is known a manner of forming a transparent and electrically conductive film on a light transmissive insulating substrate, etching said transparent and electrically conductive film to have a desired pattern suitable for a solar cell, forming an amorphous silicon semiconductor layer as a photoelectric conversion layer on the patterned transparent and electrically conductive film, and forming a back side electrode thereon. Besides this, there is also known a manner of forming an amorphous silicon semiconductor layer as a photoelectric conversion layer on a metal substrate, forming a transparent and electrically conductive film on said semiconductor layer, etching said transparent and electrically conductive film into a desired pattern, and forming a grid electrode as a collecting electrode on the patterned transparent and electrically conductive film. The latter is advantageous in that the resultant solar cell with the metal substrate can be readily processed into a configuration having a bent portion, and electrochemical treatment for repairing a defective portion can be easily conducted because the substrate of the solar cell is comprised of a metal, and continuous film formation can be conducted.
A chemical etching method of selectively etching a transparent and electrically conductive film formed on a substrate to have a desired pattern in the production of a solar cell is known (see, Japanese Unexamined Patent Publication No. 108779/1980 and U.S. Pat. No. 4,419,530). Herein, such chemical etching method will be described.
In a first step, a photoresist (comprising a printing ink or resin) is formed on a transparent and electrically conductive film formed on a substrate by means of printing technique such as silk screen printing or flexographic printing or a spinner, and the photoresist is subjected to light exposure with a desired pattern, followed by subjecting to development, whereby forming a desired positive resist pattern. In a second step, a negative portion (excluding the positive resist pattern) comprising an exposed portion of the transparent and electrically conductive film is etched with an etching solution containing ferric chloride or nitric acid so that a portion of the transparent and electrically conductive film situated under the positive resist pattern remains. In this case, the negative portion may be removed by means of dry etching such as plasma etching so that the portion of the transparent and electrically conductive film situated under the positive resist pattern remains. In a third step, the positive resist pattern (comprising a cured photoresist pattern) remaining on the transparent and electrically conductive film is removed by eluting it with a releaser, peeling it, or subjecting it to dry processing by means of plasma ashing, to thereby form a desired pattern of the transparent and electrically conductive film.
There is also known an electrochemical etching method of etching a transparent and electrically conductive film used in liquid crystal display or EL elements, wherein a transparent and electrically conductive film is formed on a substrate, a resist pattern is contacted with the surface of the transparent and electrically conductive film, the resultant is immersed in an electrolyte solution comprising an aqueous solution of HCl followed by energization with an electric current, whereby an exposed portion of the transparent and electrically conductive film which is not covered by the resist pattern is patterned (see, Japanese Unexamined Patent Publication No. 290900/1987).
Incidentally, in the case of a thin film solar cell such as an amorphous silicon solar cell, there is sometimes a problem in that the output of the voltage and electric current is markedly decreased due to short circuit defects created during the formation of its semiconductor layer. Such short circuit defects are usually created in the case where defects such as pinholes of electrically connecting the upper and lower electrodes to the semiconductor layer are present. There is a tendency that such short circuit defects increase as the size of the solar cell is enlarged.
In view of this, in the case where a large area solar cell is continuously fabricated, for instance, by way of the so-called roll-to-roll process, after the formation of the semiconductor layer or transparent and electrically conductive film, it is necessary to eliminate the short circuit defects which are possibly present therein.
U.S. Pat. No. 4,166,918 discloses a method of removing short circuit defects created during the fabrication process of a solar cell. This method is to burn out the short circuit defects present in the solar cell by applying a reverse bias voltage of sufficient magnitude which is less than the breakdown voltage of the solar cell. Besides this, U.S. Pat. No. 4,729,970 discloses a method of passivating short circuit defects in a solar cell by applying a reverse bias voltage to the solar cell in an electrolyte solution to remove a transparent and electrically conductive film composed of ITO indium tin oxide or the like present at the peripheries of the short circuit defects whereby passivating the short circuit current current paths.
In general, in the case of fabricating a large sized solar cell, there is often employed a manner of patterning the transparent and conductive film to have a desired size by subjecting the transparent and conductive film to etching treatment, removing the short circuit defects, and forming a collecting electrode. In accordance with this method, it is possible to fabricate a large area thin film solar cell having a high performance.
Now, any of the conventional patterning processes by way of the etching treatment is problematic in that it involves a variety of steps, i.e., the formation of a positive resist pattern using a photoresist, exposure, development, etching, resist removal, and the like, and they have such problems as will be described below.
For the conventional chemical etching process, there are problems such that because the etching treatment is conducted in an electrolyte solution, the expansion or removal of a resist is liable to occur and therefore, it is difficult to attain precise etching. In addition, it is necessary to precisely control not only the temperature for the etching solution but also the period of time during which the etching treatment is conducted.
For the conventional dry etching process, although it is possible to attain the patterning at a high precision, there are problems such that the treatment speed is slow and the throughput of the apparatus used is low, resulting in increased production cost, and in addition, since a strong oxidant is used, specific handling steps are necessary for it and also the waste liquid.
For the conventional plasma ashing process, although it is free of environmental pollution because no solution is used, there is a problem in that it cannot be employed for all the resists.
For the conventional electrochemical etching process, although it is advantageous in that the temperature of the etching solution need not be as precisely controlled as in the case of the foregoing chemical etching process, there are drawbacks such that to obtain a desired etched pattern, a given resist pattern is necessary to be in close-contact with the surface of a transparent and electrically conductive film formed on a substrate, no patterning can be conducted without the presence of a photoresist, and the step of forming a pattern is essential.
In the case of patterning a transparent and electrically conductive film formed on a stacked body comprising a plurality of thin film layers being stacked as a semiconductor layer for a photovoltaic element or the like by means of the conventional chemical or electrochemical etching process, there are such problems as will be described in the following. That is, when the treatment period of time with the etching solution is long, negative influences are liable to effect to the stacked body; and when the control of the temperature of the etching solution is not sufficiently conducted, defectively etched portions are liable to occur, resulting in electrical shorts or shunts in the resulting photovoltaic element in the etching process using the resist, there are also problems. when resist separation occurs during the etching process, overetching often occurs to unnecessarily etch a portion, resulting in exterior defects for a photovoltaic element and/or in making the resulting photovoltaic element to have inferior characteristics; and in this case; there is a tendency of damaging a region or layer which is not to be etched.
In the patterning process by the etching treatment by way of in the fabrication of a solar cell, patterning is usually conducted through a photoresist or the like by means of the chemical etching process, dry etching process, or electrolytic etching. And in any case, a variety of steps, i.e., close-contact of a photoresist pattern to a solar cell substrate, etching, removal of the photoresist, rinsing, washing, drying, and the like are necessary. And after the completion of these steps, a further step of removing short circuit defects possibly present in the solar cell substrate is often conducted.
In accordance with the roll-to-roll process, it is possible to continuously form a semiconductor layer, a transparent, electrically conductive film and the like. However, in practice, it is difficult to make the etching process and the short circuit defects-removing process so that they can be continuously conducted. And the etching steps involve a variety of treatment steps as above described. Therefore, even in the case where the roll-to-roll process is employed, there are problems such that it is difficult to attain the fabrication of a defect-free solar cell at a high yield and with a reduced production cost. In addition to this, because the etching process and the short circuit defects-removing process are separately conducted, in the case of mass-producing a solar cell, there is an increased possibility that physical defects will arise.
Therefore, there is an increased demand for making an improvement in the conventional process for fabricating a solar cell so that the etching process and the short circuit defects-removing process can be continuously conducted.
Incidentally, the patterning in accordance with conventional etching process using an etching apparatus is conducted, for example, in the manner shown in FIG. 1(a) through FIG. 1(h). Particularly, first, on a transparent and electrically conductive film 202 formed on a metal substrate 201 (see, FIG. 1(a)), a resist 204 is formed using a coating device 203 (see, FIG. 1(b)), followed by drying using a drier (not shown). Thereafter, a mask pattern 205 is arranged on the resist 204, followed by subjecting the resultant to light exposure using a light filter 206 (see, FIG. 1(c)). Successively, development is conducted in a vessel 207 containing a developer (see, FIG. 1(d)). After the development treatment, washing is conducted in a washing and rinsing vessel 208 (see, FIG. 1(e)), followed by drying. Thereafter, the body is subjected to etching treatment in an etching vessel 209 containing an etching solution (see, FIG. 1(f)), followed by removing the resist in a resist removing vessel 210, and successively followed by washing and then drying (see, FIG. 1(g)), where the transparent and electrically conductive film 202 on the metal substrate 201 is patterned as shown in FIG. 1(g). FIG. 1(h) is a schematic slant view illustrating a product having a desired pattern thus obtained.
In the above photolithography patterning process comprising the foregoing numerous steps, these steps are extremely difficult to combine, and the respective instruments in which the respective steps are conducted are also extremely difficult to integrate. If some of these steps could be designed so that they can be conducted in the same instrument, the instrument is unavoidably large. Hence, it is extremely difficult to realize such a system that these numerous steps can be continuously conducted to complete the patterning process within a short period of time.
This situation is more or less the same also in the fabrication of a solar cell. Particularly, the etching process in the fabrication of a solar cell involves numerous steps similar to those described in the above, and these numerous steps and the respective instruments in which the respective steps are conducted are extremely difficult to combine. Therefore, using separate instruments each for conducting each step is unavoidable. In addition, it is extremely difficult to realize an apparatus which is capable of conducting both the etching process involving the numerous steps and the short circuit defects-removing process in the apparatus. If the etching process and the short circuit defects-removing process could be designed so that they can be conducted in one apparatus, the apparatus is unavoidably large. Hence, it is extremely difficult to realize such a system that the etching process and the short circuit defects-removing process can be continuously conducted within a short period of time.